Hydrogen storage material and manufacturing method of the same

ABSTRACT

A hydrogen storage material ( 1 ) having excellent hydrogen storage capability and having such a low hydrogen desorption temperature as not to significantly hinder the use thereof, and also capable of being mass-produced, and a manufacturing method of the same can be obtained. The hydrogen storage material has a layered deformation structure including plastic deformation, and one layer ( 2 ) of the layered deformation structure is formed from an alloy or compound including an element of groups 2A, 3A and 4A or an element of at least one of the groups 2A, 3A and 4A, and another layer ( 3 ) being in contact with the one layer is formed from an alloy or compound including an element of groups 6A, 7A and 8A or an element of at least one of the groups 6A, 7A and 8A.

TECHNICAL FIELD

The present invention relates to a hydrogen storage material, and moreparticularly, relates to a hydrogen storage material having excellenthydrogen storage capability and also a reduced hydrogen desorptiontemperature, and a manufacturing method of the same.

BACKGROUND ART

With growing interest in the hydrogen energy systems, research anddevelopment of the hydrogen storage materials have been activelyconducted searching for materials for use in storage and transport ofhydrogen, separation and refinement of hydrogen gas, energy conversionapparatuses, and the like. The research and development has shown thatthe hydrogen storage materials subjected to repeated hydrogen absorptionand desorption are pulverized in a crumbling manner. Thus, materialshaving excellent hydrogen storage capability and also being highlyresistant to pulverization resulting from the repeated absorption anddesorption of hydrogen have been strongly demanded. In response to this,a proposal has been made to recommend a material having a thin-filmlaminated structure formed from a group 4A metal and any one of thegroup 6A, 7A and 8A metals (Japanese Laid-Open Publication No. 9-59001).Such a laminated, thin film body has a highly increased resistance topulverization resulting from absorption and desorption of hydrogen.Moreover, since the group 4A metals having an hcp structure in the stateof a bulk material have a bcc structure in the thin-film, laminatedstructure, the number of interstitial sites that may store hydrogen isincreased. Since the group 4A metals originally have strong bondingpower with hydrogen and thus have high hydrogen absorbing capability,the increased interstitial site density results in increased hydrogenstorage capability. Accordingly, materials being less susceptible topulverization and having extremely high hydrogen storage capability canbe obtained from the above-mentioned material having a thin-film,laminated structure formed from a group 4A metal and any one of thegroup 6A, 7A and 8A metals.

However, the above-mentioned thin-film, laminated material includes agroup 4A element, Ti, and therefore is heavy in weight. Moreover, massproduction of the thin-film, laminated material is restricted in termsof resources, thereby necessarily making the material highly expensivebeyond the price suitable for practical use, as an industrial materialof this type. Accordingly, an element alternative to the group 4A metalshad been sought. As a result, it was found that the group 2A and 3Ametals have the capability similar to that of the group 4A metals interms of the hydrogen storage capability, and a hydrogen storagelaminated material was proposed which has a group 2A or 3A metalsubstituted for a group 4A metal (Japanese Patent Application No.11-165890). For example, Mg of the group 2A elements is rich inresources and also light in weight. Therefore, it has become possible toobtain an inexpensive, lightweight laminated material being lesssusceptible to pulverization and also having excellent hydrogen storagecapability.

It is an object of the present invention to provide a hydrogen storagematerial having high hydrogen storage capability and also having such alow hydrogen desorption temperature as not to significantly hinder thedaily, easy use of the nickel-hydrogen secondary batteries,hydrogen-utilizing fuel cells, hydrogen-utilizing energy conversionsystems and the like, and more specifically, as low as 150° C. or less,and capable of being mass-produced, and a manufacturing method of thesame.

DISCLOSURE OF INVENTION

A hydrogen storage material of the present invention includes a layereddeformation structure formed in a starting material subjected to plasticdeformation, wherein one layer of the layered deformation structure isformed from an alloy or compound including an element of groups 2A, 3Aand 4A or an element of at least one of the groups 2A, 3A and 4A, andanother layer being in contact with the one layer is formed from analloy or compound including an element of groups GA, 7A and 8A or anelement of at least one of the groups 6A, 7A and 8A.

With this layered deformation structure, contact between one layer andanother layer is assured, and one layer is likely to include a bcccrystal structure, whereby the interstitial site density for storinghydrogen can be increased. Moreover, since the layered deformationstructure is realized by plastic deformation, defects such asdislocations and lamination defects are formed at a high density, sothat hydrogen is trapped in the defect portions, resulting in improvedhydrogen storage capability. Moreover, since the defect portions serveas a fast hydrogen diffusion path, formation of the defect portions at ahigh density significantly reduces the hydrogen desorption temperature.In addition, since the hydrogen storage material can be manufactured byprocessing means such as rolling, a practically required amount on theorder of tons can be produced in a short period with high efficiency.

Note that, the layered deformation structure refers to the structureformed from laminated dissimilar materials subjected to strongdeformation working involving plastic deformation as shown in FIGS. 1and 2, and is different from the structure shown in FIG. 3 that isconventionally known as a laminated structure. In the structure shown inFIG. 1, each layer extends uniformly in the rolling, wire-drawingdirection, whereas in the structure shown in FIG. 2, a portion whereeach layer extends uniformly is randomly folded.

In the case where fast diffusion of the hydrogen atoms in the defectportions is important, or otherwise, the above-mentioned hydrogenstorage material has a defect density resulting from such strongdeformation working that causes a half-band width of at least one ofmain diffraction peaks in an X-ray diffraction pattern of the layereddeformation structure to be 0.2° or more.

The density of defects such as dislocations and lamination defects canbe evaluated by the half-band width of an X-ray diffraction peak.Normally, in order to increase the hydrogen diffusion velocity, thehalf-band width is 0.2° or more, preferably 0.5° or more, and mostpreferably 1° or more. Impurity segregation is likely to occur in thedefect portions, and such impurity segregation results in biasedcharges. These biased charges are considered to have a function toattract and trap hydrogen. In order to clearly induce this hydrogentrapping function, the half-band width is preferably 0.5° or more.However, it is not necessarily desirable to increase the half-bandwidth, and it is not preferable that the strong deformation workingcauses an amorphous state, i.e., the state where X-ray diffraction doesnot have clear diffraction peaks. In the amorphous state, the bondstructure forming the crystal structure is disconnected, and thehydrogen atoms are strongly trapped in this disconnected bond structure.Therefore, the hydrogen storage capacity is increased, but the amount ofhydrogen capable of being desorbed at a practical temperature issignificantly reduced. Note that the main diffraction peaks refer to thehighest three peaks among the diffraction peaks of a material that is tobe subjected to the X-ray diffraction. Alternatively, in the case of amaterial having many diffraction peaks, the main diffraction peaks mayrefer to the highest five peaks, instead of the highest three peaks. Ahalf-band width can be easily read on the chart. However, a diffractionpeak that already has a half-band width of 0.2° or more before plasticworking is excluded from the measurement. Alloys having precipitationsproduced therein and the like have a diffraction line with a half-bandwidth of 0.2° or more. Accordingly, such a diffraction line is excludedfrom the measurement.

In the case where it is important in the above-mentioned hydrogenstorage material to assure a large contact area between one layer andanother layer and obtain a high defect density, or otherwise, one layerof the layered deformation structure has a thickness of 10 nm or less.

Reduction in thickness of one layer of the layered deformation structuremeans strong deformation working involving plastic deformation, and alsomeans elimination of the automorphic function due to the reducedthickness, i.e., elimination of the capability to form an originallystable crystal structure (such as a phenomenon of easy phase transitionfrom hexagonal to cubic), formation of a fast hydrogen diffusion pathand production of high-density defects serving as a hydrogen trappingsource. Moreover, a large contact area can be assured between one layerand another layer. More specifically, the thickness of one layer of thelayered deformation structure can be used as an index of a high defectdensity and large contact area. This thickness indicates an averagethickness of the thickest portions of each layer. In the case where twoone layers are continuously formed, measurement is conducted consideringthat these two layers are separated at the surface of one of the twolayers. The thickness exceeding 10 nm results in an insufficient defectdensity and contact area, and thus results in an insufficient hydrogenstorage capacity, so that the hydrogen desorption temperature exceedsthe practical value. A thin film sample is obtained which has the crosssection in the direction perpendicular to the processing direction asits surface. The thickness of the sample is measured with a transmissionelectron microscope (TEM) for ten fields or more, at least at tenpositions per field, whereby an average thickness of one layers isobtained. In the case of the powder subjected to mechanical alloyinghaving an unclear processing direction, the above-mentioned thickness isobtained by statistical processing of the measurements of ten fields ormore, at ten positions or more per field.

A method for manufacturing a hydrogen storage material according to thepresent invention includes the step of conducting strong deformationworking involving plastic deformation to a starting material including:one or more materials selected from alloys or compounds including anelement of groups 2A, 3A and 4A or an element of at least one of thegroups 2A, 3A and 4A; and one or more materials selected from alloys orcompounds including an element of groups 6A, 7A and 8A or an element ofat least one of the groups 6A, 7A and 8A.

The strong deformation working involving plastic deformation introducesdefects such as dislocations and lamination defects into the crystallattice. As the number of times of the processing is increased, thedislocations, lamination defects and the like transition into the statewhere the defects are accumulated in a tangled manner. Thesedefect-accumulated portions improve the hydrogen diffusion velocity, andfunction as a fast hydrogen diffusion path. Moreover, impuritysegregation is likely to occur in the defects, and such impuritysegregation induces biased charges. The biased charge portions functionas hydrogen-atom trapping sites, and therefore increase the hydrogenstorage capacity itself. Moreover, the strong deformation workinginvolving plastic deformation increases the contact area between onelayer and another layer in the layered deformation structure, and onelayer is likely to include a bcc structure. If the bcc structure isincluded, the density of interstitial sites for storing hydrogen atomsis increased, whereby the hydrogen storage capability is enhanced.

In the above-mentioned method for manufacturing a hydrogen storagematerial, in the case where it is important to increase the defectdensity to a prescribed value or more and to sufficiently reduce thethickness of each constituent layer of the layered deformationstructure, or otherwise, the material subjected to the strongdeformation working involving plastic deformation is again laminated toform a starting material, and the starting material is further subjectedto strong deformation working involving plastic deformation.

The above-mentioned method allows the material to be effectivelysubjected to large plastic deformation.

In the above-mentioned method for manufacturing a hydrogen storagematerial, in the case where the layered deformation structure is notlikely to be formed after the material was formed into a plate or lineshape, or otherwise, the starting material is in a form of powder orpellet. The starting material is enclosed in a ductile pipe, and theresultant pipe is formed into a plate or line shape, and subjected tothe strong deformation working. Alternatively, the strong deformationworking involving plastic deformation is conducted by mechanicalalloying.

The mechanical alloying causes the layered deformation structure to beformed at the surface or into the inside of the material in the powderstate. Therefore, a preforming body can be made from this powder, andprocessed into a desired shape. As a result, a component having bothhigh hydrogen storage capability and low hydrogen desorption temperaturecan be obtained without restriction on the shape.

In the above-mentioned method for manufacturing a hydrogen storagematerial, in the case where the temperature rises or annealing isconducted during plastic deformation, or otherwise, the strongdeformation working involving plastic deformation, or annealing isconducted in a temperature range corresponding to 80% or less of amelting point of a material selected as the starting material.

If the temperature exceeds 80% of the melting point, alloying progressesbetween the dissimilar layers, whereby the intention to make thedissimilar materials in contact with each other is hindered. The meltingpoint as used herein generally indicates the melting point of a materialhaving a higher melting point out of the dissimilar materials. However,in the case where a high-melting-point material is selectively diffusedinto a low-melting-point material, the melting point of thelow-melting-point material may be possible in order to reduce thediffusion driving force in terms of prevention of the progress inalloying. Moreover, if the temperature exceeds 80% of the melting point,the density of dislocations and lamination defects is reduced, so thatthe hydrogen storage capability is not degraded and also the hydrogendesorption temperature is not reduced. Note that the melting point isindicated in centigrade ° C., and the above-mentioned temperature rangeindicates a temperature in centigrade ° C. that is equal to or lowerthan 80% of the melting point in centigrade ° C.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing one layered deformation structurerecognized in a hydrogen storage material of the present invention.

FIG. 2 is a schematic diagram showing another layered deformationstructure recognized in a hydrogen storage material of the presentinvention.

FIG. 3 is a schematic diagram showing a conventional laminatedstructure.

FIG. 4 is a schematic diagram showing the structure of an apparatus forrealizing hydrogen storage treatment.

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment 1: Rolling)

First, Embodiment 1 will be described in which a layered deformationstructure according to the present invention is formed by rolling.Combinations of one of the group 2A, 3A and 4A metals and one of thegroup 6A, 7A and 8A metals as shown in Present Example Nos. 1 to 12 ofTable 1 were used. One of the group 2A, 3A and 4A metals is hereinreferred to as one layer, and one of the group 6A, 7A and 8A metals isherein referred to as another layer, but one layer and another layer arenot necessarily alternately, regularly arranged. The arrangement is notregular in a portion where one layer is severed due to strongdeformation working. In the following description, the group 2A, 3A and4A metals are referred to as L-type metals, and the group 6A, 7A and 8Ametals are referred to as H-type metals. Examples such as combinationsof layers respectively formed from two of the H-type metals and a layerformed from one of the L-type metals which is interposed therebetween,as shown in Present Examples Nos. 13 to 17, were used. For example, forNo. 13, the combination shown in Table 1 is repeated, so that the H-typemetal layers are in contact with each other at the boundary between therepeated combinations. Among Nos. 13 to 17, Present Example No. 15 isformed from layers of two metals: an L-type metal and an H-type metal,wherein Ms-Si compound (magnesium silicide) is used as the L-type metal.

Hereinafter, a method for manufacturing specimens of Present ExamplesNos. 1 to 12 will be described. A plate of a single L-type metal havinga thickness of 1 mm was pickled, and then annealed in high temperaturevacuum so as to sufficiently remove a surface oxide film into a cleansurface. The step of removing the surface oxide film may be conductedonly by pickling, or may be conducted by annealing in hydrogen or Aratmosphere without conducting pickling. Alternatively, the surface oxidefilm may be removed by machine cutting. Then, a plate of a single H-typemetal having a thickness of 1 mm was subjected to the same treatment,and then a surface oxide film was similarly removed into a cleansurface. Thereafter, the resultant surface-cleaned plates arealternately laminated one by one into a laminated body of 20 plates intotal (nominal thickness: 20 mm). Using this laminated body as a rollingmaterial, multi-pass rolling was conducted with a rolling reductionratio of 10% to 50% per pass. Heating was not conducted before rolling,and the processing speed and inter-pass time interval were adjusted suchthat the temperature did not exceed 300° C. during rolling due to theprocessing heat.

The thickness of the laminated body was measured after each roll pass.At the thickness of 10 mm, the laminated body was severed at the centersuch that the length in the rolling direction was halved. Impuritiessuch as surface lubricating oil adhering to the surface of eachlaminated body were removed by pickling and annealing in hightemperature vacuum. These halved, surface-cleaned laminated bodies werefurther laminated into a rolling material having a nominal thickness of20 mm, and rolling was again repeated. This process of rolling,severing, surface cleaning and formation of the laminated body as arolling material were repeated 15 times or more in total, or 20 times ormore depending on the specimens, so that the thickness of each layerfell in the range of 1 to 10 nm. Note that two laminated bodies eachhaving a thickness of 10 mm before lamination were laminated this time.Practically, however, the thickness may be greater than or smaller than10 mm, and the same effects can be obtained even if two or morelaminated bodies are laminated. In the case where significant breakageoccurred during rolling, annealing was conducted so as to improveprocessability. However, the annealing temperature must be equal to orlower than the temperature that does not induce diffusion between theL-type metal layer and the M-type metal layer, for example, 300° C. orless.

TABLE 1 Hydrogen XRD Peak Material Storage Capacity Showing bccCombination (H/M) Structure Defects Present Example  1 Ti/Cr 3.0 ExistExist  2 Ti/Ni 2.5 None Exist  3 Ti/Fe 2.5 Exist Exist  4 Mg/Cr 2.5Exist Exist  5 Mg/Ni 2.5 None Exist  6 Ca.Mg/Cr 2.5 Exist Exist  7 Y/Ni3.0 None Exist  8 Y/Cr 2.5 Exist Exist  9 La/Ni 2.5 None Exist 10 La/Cr2.5 Exist Exist 11 Yb/Ni 2.5 Exist Exist 12 Yb/Cr 2.5 Exist Exist 13Cr/Mg/Ni 3.0 Exist Exist 14 Ti/Mg/Ni 3.0 None Exist 15 Mg—Si/Ni 2.0Exist Exist 16 Mg/Cr/Ti 2.5 Exist Exist 17 Fe/Mg/Cr 2.0 Exist ExistComparative Example 18 MgNi₅ 1.5 None None 19 CaNi₅ 1.0 None None 20LaNi₅ 1.0 None None 21 LaCr₅ 0.5 None None

The specimens of Present Example Nos. 13 to 17 of Table 1 are basicallymade by the same method as the above-described rolling method exceptthat the laminated body as a rolling material is formed from three typesof thin plates. However, Present Example No. 15 is formed from two typesof thin plates, one of them being Mg-Si compound (magnesium silicide). Athin film was cut out as a hydrogen storage specimen from the specimenmade by the above-described method. This hydrogen storage specimen wassubjected to hydrogen storage treatment by an electrolytic chargemethod. An apparatus for conducting the hydrogen storage treatment isshown in FIG. 4. Referring to FIG. 4, in conducting the hydrogen storagetreatment, a specimen 10 was soaked in a 0.1 M NaOH solution and a Ptcounter electrode 12 was soaked in a 0.5 M K₂SO₄ solution. A negativecurrent was applied to the specimen 10, whereas a positive current wasapplied to the Pt counter electrode 12, both for a predetermined timeperiod by means of a constant-current power supply 11. TR6120A made byAdvantest was used as the constant-current power supply 11. Note thatthe current value was basically 10 mA, and the current application timewas set to one hour. A value as given by current (A)×time (s)corresponds to the quantity of electricity, and this value was used tocalculate the hydrogen generation amount by the electrolysis based onFaraday's law. Measurement of stored hydrogen was conducted with EMGA621made by Horiba. This apparatus is capable of conducting any one ofhydrogen absolute quantity analysis and temperature-programmed analysis.Specifically, the hydrogen storage capacity was obtained by thefollowing method: first, the specimen is warmed up, and hydrogen leavingthe specimen is quantified by gas analysis. Subsequently, the specimenhaving discharged hydrogen is dissolved in acid, and the specimen isquantified by chemical analysis. H/M was obtained from both quantitativevalues. Bulk materials were used in comparative examples.

A sample of the thickness cross section of the plate subjected to thefinal rolling was cut out, and a thin film was made therefrom fortransmission electron microscopic observation (TEM). Moreover, an X-raydiffraction pattern for the thickness cross section was obtained by a 2θmethod and analyzed. One purpose of obtaining the X-ray diffractionpattern is to examine whether or not an L-type metal having an hcpstructure in the bulk material partially includes a bcc structure in thelayered deformation structure. Another purpose is to measure thehalf-band width of an appropriate diffraction peak and thereby determinewhether or not the high-density defects such as dislocations orlamination defects introduced by plastic working such as rolling stillremain in the final state. In Table 1, defect determination wasconducted as follows: for the samples prior to the hydrogen storagetreatment, the half-band width of a clearly recognized, attributableX-ray diffraction peak was measured. The samples having a half-bandwidth of 1° or more are denoted with “Exist” in the column “Defects”,whereas the samples having a half-band width less than 1° are denotedwith “None”.

The test result is shown in Table 1. Every sample of the PresentExamples in Table 1 had a layered deformation structure as shown in FIG.1 or 2 within the same sample. According to the TEM observation, therespective thicknesses of the L-type metal layer and H-type metal layerwere not uniform in the plate thickness direction, but were reduced tothe range of 10 nm to 1 nm. Referring to FIG. 1, a layered deformationstructure 1 includes one layer 2 including an L-type metal element andanother layer 3 including an H-type metal element, and each of thelayers typically uniformly extend in the processing direction. One layerand another layer are alternately arranged in some portions, but are notnecessarily alternate. Another layer is continuously formed in aseparated part 4 of one layer. Similarly, one layer is continuouslyformed in a separated part 5 of another layer. In a part of the sample,a portion having a clear processing direction is folded as shown in FIG.2, so that the random crystal grains assemble together as a whole. Thestructure shown in FIG. 2 was often recognized in a portion processed toa high processing degree, such as the ends of the sample, but is notlimited to the portion processed to a high processing degree. Thelayered deformation structure as recognized in the samples of thePresent Examples is different from the conventional orderly structurehaving a uniform thickness shown in FIG. 3. In FIG. 3, a laminatedstructure 101 is comprised of one layer 102 and another layer 103 bothhaving a uniform thickness. Although these layers extend uniformly inthe processing direction with a uniform thickness, such orderlyarrangement is not always desirable. For example, the structure shown inFIG. 2 has an improved hydrogen moving speed because the directionalityis lost as a whole.

All of Present Examples Nos. 1 to 17 had H/M of 2.0 or more. Nos. 7 to12 and No. 15 also had H/M of 2.0 or more. Among others, PresentExamples Nos. 2, 5, 7, 9 and 14 have a high H/M value because ofhigh-density defects, despite that they do not have a bcc structure. Thereason for this is as follows: as described above, impurity segregationis likely to occur in the vicinity of the defects, and such impuritysegregation induces biased charges in the vicinity thereof, so thathydrogen atoms are trapped therein.

In the case where strong rolling was conducted like Embodiment 1, amultiplicity of defects are introduced into the crystal. The defectsthus introduced transition to a tangled state as the number ofprocessing repetitions is increased. These defects not only improve thehydrogen storage capability as described above, but also serve as a fasthydrogen diffusion path and reduce a hydrogen desorption temperature.The defect density is evaluated by the half-band width of an X-raydiffraction peak. However, since a microcrystalline material on theorder of nanometers is not likely to have dislocations in the crystalgrains, the defect density as used herein does not necessarily indicatethe dislocation density. In order to increase the hydrogen diffusionvelocity, the half-band width is 0.2° or more. In order to furtherincrease the hydrogen storage capacity, the half-band width of 0.50 ormore is desirable. The Present Examples as denoted with “Exist” in thecolumn “Defects” in Table 1 have a half-band width of 1° or more, andtherefore significantly contribute to improvement in H/M.

In addition, Present Example No. 5 of Table 1 (Mg/Ni (molar ratioMg:Ni=2:1)) was subjected to the temperature-programmed analysis usingthe above-mentioned EMGA621 made by Horiba, so that the hydrogendesorption rate was obtained under the heating condition of theprogramming rate of 10° C./min, and compared with that of a bulkmaterial Mg₂Ni. In Present Example No. 5, hydrogen desorption wasrecognized from 50° C., and the desorption rate was maximized at 100° C.On the other hand, the bulk material Mg₂Ni desorbed hydrogen at atemperature of 200° C. to 300° C. Accordingly, by increasing the defectdensity by such strong deformation working that causes the half-bandwidth of an X-ray diffraction peak to be 1° or more as described above,the hydrogen storage material according to the present invention can bepractically used as a hydrogen supply source for the nickel-hydrogensecondary batteries and fuel cells. Unlike a manufacturing method usingan ion plating method, this rolling manufacturing method is suitable formass production, and enables the hydrogen storage material of thepresent invention to be manufactured on the order of tons with highproductivity.

(Embodiment 2: Wire Drawing)

First, Embodiment 2 will be described in which a laminated deformationstructure according to the present invention is formed by wire drawing.Combinations of one of the L-type metals and one of the H-type metals asshown in Present Example Nos. 1 to 12 of Table 2 were used. Moreover,examples such as combinations of two of the H-type metals and one of theL-type metals as shown in Present Examples Nos. 13 to 17 were used.However, Present Example No. 15 is formed from plates of two metals: anL-type metal and an H-type metal, wherein Mi-Si compound (magnesiumsilicide) is used as the L-type metal.

TABLE 2 Compo- sition Hydrogen Ratio Storage XRD Peak Material (MolarCapacity Showing bcc Combination Ratio) (H/M) Structure Defects PresentExample  1 Ti/Cr 1:1 3.0 Exist Exist  2 Ti/Ni 2:1 2.5 None Exist  3Ti/Fe 1:1 2.5 Exist Exist  4 Mg/Cr 2:1 2.5 Exist Exist  5 Mg/Ni 2:1 2.5None Exist  6 Ca.Mg/Cr 1:1:2 2.5 Exist Exist  7 Y/Ni 1:1 2.0 None Exist 8 Y/Cr 1:1 2.5 Exist Exist  9 La/Ni 1:1 2.5 None Exist 10 La/Cr 1:1 2.5Exist Exist 11 Yb/Ni 2:1 2.5 Exist Exist 12 Yb/Cr 2:1 2.5 Exist Exist 13Cr/Mg/Ni 1:2:1 2.0 Exist Exist 14 Ti/Mg/Ni 1:1:1 2.0 None Exist 15Mg—Si/Ni 2:1:1 2.0 Exist Exist 16 Mg/Cr/Ti 1:1:2 2.5 Exist Exist 17Fe/Mg/Cr 1:1:1 2.0 Exist Exist Comparative Example 18 MgNi₅ — 1.5 NoneNone 19 CaNi₅ — 1.0 None None 20 LaNi₅ — 1.0 None None 21 LaCr₅ — 0.5None None

Hereinafter, a method for manufacturing the specimens of Present ExampleNos. 1 to 12 in Table 2 will be described. Powders of the L-type andH-type metals were prepared at each molar ratio shown in Table 2, andsufficiently uniformly mixed. A copper pipe having an outer diameter of20 mm and inner diameter of 16 mm was loaded with the resultant mixture.The diameter of each powder was 1 mm or less. The size of the powders isdesirably 50 μm or less. Thereafter, the end of the copper pipe wasclosed, and subjected to wire drawing. The wire drawing was conductedwith a roller die, but swaging, drawing with a hole die, or rolling mayalso be possible. The wire drawing was conducted at the area reductionratio of 5% to 30% per pass, and the total area reduction ratio was 96%or more. The total area reduction ratio is preferably 99.5% or more. Theheat treatment at 300° C. or less during wire drawing would facilitatethe processing, and also improve the density due to increased adhesionbetween the powders. After the processing, copper on the surface of thewire was removed by chemical treatment or machining, whereby the wireformed from the mixed body of the L-type and H-type metals was obtained.Hydrogen specimens for measuring the hydrogen storage capability H/Nwere cut out from the wires. In addition, X-ray diffraction specimenswere made as follows: the cross section of a bundle of a plurality ofwires was cut out and embedded into a resin, and the resultant resin waspolished. The H/M measurement method and X-ray diffraction method wereconducted in the same manner as that of Embodiment 1.

According to the test result shown in Table 2, the Present Examples havea high H/M value of 2.0 or more. Among these, Nos. 2, 5, 7, 9 and 14have a high H/M value, despite that they do not include a bcc structurein the L-type metal layer. As specifically described in Embodiment 1,this is because of a high defect density. More specifically, in thePresent Examples, the half-band width of an X-ray diffraction peak is 1°or more, and therefore dislocations, lamination defects and the like areformed at a high density. On the other hand, the bulk materials ofComparative Example Nos. 18 to 21 have H/M in the range of 0.5 to 1.5.Moreover, Comparative Example Nos. 18 to 21 are susceptible topulverization as a result of repeated hydrogen absorption and desorptioncycles, because they are bulk materials.

In order to understand the effects of the above-mentioned high defectdensity on the hydrogen desorption temperature, Present Example No. 5 ofTable 2 was examined for hydrogen desorption. The examination method isthe same as that described in Embodiment 1. As a result, it was foundthat Present Example No. 5 of Table 2 starts hydrogen desorption from50° C. and has the maximum desorption rate at 100° C. On the other hand,a bulk material Mg₂Ni desorbed hydrogen at a temperature of 200° C. to300° C. Accordingly, it was found that the wire drawing also introducesthe high-density defects into the hydrogen storage material, and reducesthe hydrogen desorption temperature by 100° C. to 200° C. As a result,the hydrogen storage material can be used as a hydrogen supply sourcefor the fuel cells and an electrode of the nickel-hydrogen secondarybatteries that are used for articles of daily use. These wires are useddirectly as wires, or are formed into sheets or the like for use as theelectrode of the nickel-hydrogen secondary batteries and the like. Theabove-described wire drawing enables the hydrogen storage material,ofthe present invention to be manufactured on the order of tons with highproductivity.

(Embodiment 3: Mechanical Alloying)

Using Mg pellets, Ni carbonyl powder and metal chromium powder asmaterials, the materials were premixed by a V-shaped mixer at the finalcomposition ratio, and then the resultant mixture was subjected tomechanical alloying (MA) for 500 hours by means of a planetary ball millwith Ar gas enclosed therein. The powder thus obtained was preformedwith a pressure of 500 MPa using a mold, heated to 300° C. and thenimmediately extruded at the extrusion ratio of 10: 1 into a bar-likematerial. In Embodiment 3, a layered deformation structure was formed atthe surface portion of each powder at the stage of mechanical alloying,and was extended by extrusion, so that the powders thus extended in theextruding direction overlap each other into a layered deformationstructure. Note that since Mg has a melting point of 651° C., Ni 1450°C. and Cr 1890° C., the above-mentioned heating temperature of 300° C.is well within 80% of the melting point of each material. Morespecifically, for Mg, the above-mentioned 80% temperature is 520.8° C.,and the heating temperature of 300° C. is lower than this value.

A hydrogen specimen was obtained from the above-mentioned bar-likematerial, and H/M and hydrogen desorption temperature were measured bythe same method as that of Embodiment 1. The specimen of Embodiment 3had H/M of 2.5. Moreover, hydrogen desorption was started around 50° C.,and the desorption rate was maximized at a temperature in the range of80° C. to 100° C. About 80% of stored hydrogen had been desorbed at thetime the temperature reached 100° C. Moreover, since the hydrogenstorage material was started from the powders and thus had beenpulverized in advance, pulverization of the hydrogen storage materialdid not significantly progress even after the repeated hydrogenabsorption and desorption.

(Embodiment 4: Cyclic Press)

Using an Mg thin plate, Ni thin plate and Cr thin plate as materials,these plates were combined at the final composition ratio, and uniformlysized and severed. Then, the end of the resultant material wassemi-fixed by spot welding. Thereafter, the resultant material waspressed at the pressing force of 800 MPa in a simple mold. The laminatedbody thus obtained was halved, and the halved laminated bodies weresurface-cleaned and then again laminated for pressing. This cycle ofpressing, severing, surface cleaning, and formation of the laminatedbody as a pressed material was repeated 1,000 times, whereby a layereddeformation structure was obtained.

In this cyclic press method, the material began to be hardenedapproximately when the number of repetition times exceeded 200. Then,the material subjected to pressing 1,000 times was kept in 99.99%-purehydrogen at 500° C. for 24 hours in order to remove oxygen within thematerial. The hydrogen storage capacity of the resultant material was3.0 in H/M, but the hydrogen absorption temperature and hydrogendesorption temperature were within 100° C. from the room temperature,and the ratio of the desorption amount to the absorption amount was 80%or more. The reason why the hydrogen absorption temperature anddesorption temperature were enabled to be within 100° C. from the roomtemperature can be considered as follows: Mg having strong affinity tohydrogen was interposed between Ni and Cr, the materials having adifferent crystal structure therefrom, so that significant latticedistortion was generated in the crystal lattice of Mg, which increasedthe bond distance between hydrogen and Mg. Note that the heatingtemperature of 500° C. in the above-mentioned hydrogen annealing islower than 520.8° C., i.e., 80% of the melting point, 651° C., of Mg.

(Embodiment 5)

The same processing material (starting material) as that of Embodiment 4was rolled at the rolling reduction ratio of 20% by a rolling mill, andsevering, surface cleaning, and formation of the laminated body wererepeated 1,000 times. The sample thus obtained was heated to 500° C. in99.99%-pure hydrogen and kept for 24 hours in order to remove oxygenwithin the sample. The hydrogen storage capacity of this sample was ashigh as 2.5 in H/M. The hydrogen absorption temperature and hydrogendesorption temperature were within 90° C. from the room temperature, andthe ratio of the desorption amount to the absorption amount was 85% ormore.

The reason why the hydrogen absorption temperature and desorptiontemperature were able to be within 90° C. from the room temperature canbe considered as follows: Mg having strong affinity to hydrogen wasinterposed between Ni and Cr, the materials having a different crystalstructure from Mg, so that significant lattice distortion was generatedin the crystal lattice of Mg, which increased the bond distance betweenhydrogen and Mg. Particularly in the case of the rolling, the processinghas directionality, so that the crystal lattice of Mg may have beenextended in the processing direction.

According to the present invention, a hydrogen storage material havinghigh hydrogen storage capability and also having a hydrogen desorptiontemperature reduced to about 100° C. can be provided by the methodcapable of mass production such as rolling. As a result, extensiveutilization in the energy-related industries becomes possible includingfuel cells and electrode materials of nickel-hydrogen secondarybatteries for driving automobiles using a large amount of hydrogenstorage material having a low hydrogen desorption temperature,hydrogen-utilizing energy conversion systems, and the like.

Although embodiments of the present invention have been described, theembodiments disclosed above are by way of illustration and example only,and the scope of the present invention is not limited to theseembodiments. The scope of the present invention is defined by theappended claims, and includes all modifications within the sense andscope equivalent to the definition of the appended claims.

What is claimed is:
 1. A hydrogen storage material, wherein the hydrogenstorage material has a layered deformation structure formed in astarting material subjected to plastic deformation, one layer (2) of thelayered deformation structure is formed from an alloy or compoundincluding an element of groups 2A, 3A and 4A or an element of at leastone of the groups 2A, 3A and 4A, and another layer (3) being in contactwith the one layer is formed from an alloy or compound including anelement of groups 6A, 7A and 8A or an element of at least one of thegroups 6A, 7A and 8A.
 2. The hydrogen storage material according toclaim 1, wherein the hydrogen storage material has a defect densityresulting from such strong deformation working that causes a half-bandwidth of at least one of main diffraction peaks in an X-ray diffractionpattern of the layered deformation structure to be 0.2° or more.
 3. Thehydrogen storage material according to claim 1, wherein the one layer ofthe layered deformation structure has a thickness of 10 nm or less.
 4. Amethod for manufacturing a hydrogen storage material, comprising thestep of conducting strong deformation working involving plasticdeformation to a starting material including: one or more materialsselected from alloys or compounds including an element of groups 2A, 3Aand 4A or an element of at least one of the groups 2A, 3A and 4A; andone or more materials selected from alloys or compounds including anelement of groups 6A, 7A and 8A or an element of at least one of thegroups 6A, 7A and 8A.
 5. The method for manufacturing a hydrogen storagematerial according to claim 4, wherein the material subjected to thestrong deformation working involving plastic deformation is againlaminated and further subjected to strong deformation working involvingplastic deformation.
 6. The method for manufacturing a hydrogen storagematerial according to claim 4, wherein the starting material is in aform of powder or pellet, and the strong deformation working involvingplastic deformation is mechanical alloying.
 7. The method formanufacturing a hydrogen storage material according to claim 4, whereinthe strong deformation working involving plastic deformation isconducted in a temperature range corresponding to 80% or less of amelting point of a material selected as the starting material.
 8. Themethod for manufacturing a hydrogen storage material according to claim4, wherein in the strong deformation working involving plasticdeformation, annealing is conducted in a temperature range correspondingto 80% or less of a melting point of a material selected as the startingmaterial.